Light controller with overload current protection circuit

Abstract

The invention provides a light controller for lamps which can reduce the number of parts, cost and size. A rectangular wave voltage output from an astable multivibrator (1) is inverted by an inverter (3) and then supplied to a gate of a P-channel MOS FET (5) for on/off driving the MOS FET (5). A current is thereby caused to flow through a lamp (7) for lighting it up. By changing a resistance value of a variable resistor (2), a cycle of the rectangular wave voltage is changed and, therefore, an amount of the current flowing through the lamp (7) is changed so as to vary the illumination intensity of the lamp.

What is claimed is: 
     
       1. A light controller for lamps comprising: a multivibrator for generating rectangular wave voltage signal;   an inverter having an input terminal connected to the multivibrator for inverting the rectangular wave voltage signal output from said multivibrator;   a P-channel MOS FET having a gate connected to receive the inverted rectangular wave voltage signal output from said inverter; and   a protective circuit connected to the gate of said MOS FET for protecting said MOS FET against an overload current;   wherein said MOS FET includes a source connected to a source voltage and a drain connected to a lamp; and   wherein said protective circuit includes a comparator having an output terminal connected the input terminal of the inverter and a temperature sensor connected to an input terminal of said comparator.   
     
     
       2. A light controller for lamps comprising: a multivibrator for generating a rectangular wave voltage signal on an output terminal;   a P-channel MOS FET having a gate connected to output terminal of the multivibrator, a source connected to a source voltage, and a drain connected to a lamp; and   a driver circuit connected to the output terminal of said multivibrator;   wherein said MOS FET is controlled by said rectangular wave voltage; and   wherein said driver circuit comprises: resistance elements each connected to a reference voltage source,   a comparator having an input terminal to which one of said resistance elements is connected, and an output terminal to which the other of said resistance elements is connected,   a first diode forward-connected between an output terminal of said multivibrator and the input terminal of said comparator, and   a second diode forward-connected to the output terminal of said comparator.     
     
     
       3. A light controller according to claim 1, wherein a second input terminal of the comparator is connected to receive a reference voltage;   wherein the temperature sensor is located adjacent the MOS FET such that heating of the MOS FET due to an overload current is detected by the temperature sensor; and   wherein the comparator prevents transmission of the inverted rectangular voltage signal in response to a detected overload current.   
     
     
       4. A light controller according to claim 3, wherein the temperature sensor is a thermistor whose resistance is determined by a temperature of the MOS FET. 
     
     
       5. A light controller according to claim 2, further comprising a protective circuit connected between the output terminal of the multivibrator and the gate of said MOS FET; wherein said protective circuit includes a second comparator having an output terminal connected the input terminal of the inverter and a temperature sensor connected to an input terminal of said second comparator.   
     
     
       6. A light controller according to claim 5, wherein a second input terminal of the second comparator is connected to receive a reference voltage;   wherein the temperature sensor is located adjacent the MOS FET such that heating of the MOS FET due to an overload current is detected by the temperature sensor; and   wherein the second comparator prevents transmission of the inverted rectangular voltage signal in response to a detected overload current.   
     
     
       7. A light controller according to claim 6, wherein the temperature sensor is a thermistor whose resistance is determined by a temperature of the MOS FET.